Cement compositions containing flexible, compressible beads and methods of cementing in subterranean formations

ABSTRACT

Cement compositions comprising flexible, compressible beads, processes for preparing such cement compositions, and methods of cementing in subterranean formations using such cement compositions. One or more flexible, compressible beads are mixed with the cement before pumping the cement into a well bore. The flexible, compressible beads are preferably composed of an elastomeric material such as a copolymer of methylmethacrylate and acrylonitrile; a terpolymer of methylmethacrylate, acrylonitrile, and dichloroethane; a styrene-divinylbenzene copolymer; and polystyrene. The flexible, compressible beads may be heated to expand the beads before mixing with the cement such that the ensuing cement composition will have a desired density. Non-flexible beads such as spherulites may also be added to the cement compositions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application of U.S. patent application Ser. No.10/350,533, filed Jan. 24, 2003 and entitled “Cement CompositionsContaining Flexible, Compressible Beads and Methods of Cementing inSubterranean Formations.”

FIELD OF THE INVENTION

This invention generally relates to cementing in subterranean formationspenetrated by well bores. More specifically, the invention relates tocement compositions comprising flexible, compressible beads, andprocesses for making the compositions.

BACKGROUND OF THE INVENTION

Well cementing is a process used in penetrating subterranean zones (alsoknown as subterranean formations) to recover subterranean resources suchas gas, oil, minerals, and water. In well cementing, a well bore isdrilled while a drilling fluid is circulated through the well bore. Thecirculation of the drilling fluid is then terminated, and a string ofpipe, e.g., casing, is run in the well bore. The drilling fluid in thewell bore is conditioned by circulating it downwardly through theinterior of the pipe and upwardly through the annulus, which is locatedbetween the exterior of the pipe and the walls of the well bore. Next,primary cementing is typically performed whereby a slurry of cement andwater is placed in the annulus and permitted to set into a hard mass(i.e., sheath) to thereby attach the string of pipe to the walls of thewell bore and seal the annulus.

Low density or lightweight cement compositions are commonly used inwells that extend through weak subterranean formations to reduce thehydrostatic pressure exerted by the cement column on the weak formation.Conventional lightweight cement compositions are made by adding morewater to reduce the slurry density. Other materials such as bentonite,diatomaceous earth, and sodium metasilicate may be added to prevent thesolids in the slurry from separating when the water is added.Unfortunately, this method has the drawback that the addition of morewater increases the cure time and reduces the strength of the resultingcement.

Lightweight cement compositions containing hollow spheres have beendeveloped as a better alternative to the cement compositions containinglarge quantities of water. The hollow spheres are typically cenospheres,glass hollow spheres, or ceramic hollow spheres. Cenospheres are hollowspheres primarily comprising silica (SiO₂) and alumina (Al₂O₃) and arefilled with gas. Cenospheres are a naturally occurring by-product of theburning process of a coal-fired power plant. Their size may vary fromabout 10 to 350 μm. These hollow spheres reduce the density of thecement composition such that less water is required to form the cementcomposition. The curing time of the cement composition is thereforereduced. Further, the resulting cement has superior mechanicalproperties as compared to cement formed by adding more water. Forexample, the tensile and compressive strengths of the cement aregreater.

During the life of the well, the cement sheath is subjected todetrimental cyclical stresses due to pressure and temperature changesresulting from operations such as pressure testing, drilling,fracturing, cementing, and remedial operations. Conventional hollowspheres suffer from the drawback of being brittle and fragile and thusoften cannot sustain those cyclical stresses. As a result, the cementsheath develops cracks and thus fails to provide zonal isolation for thelife of the well. A need therefore exists to develop a less brittlecement having properties that would enable it to withstand pressure andtemperature fluctuations for the life of the well. The present inventionadvantageously provides cement compositions that can withstand thecyclical stresses that occur during the life of the well.

SUMMARY OF THE INVENTION

The present invention includes cement compositions comprising flexible,compressible beads, a process for preparing such cement compositions,and methods for cementing a well bore in a subterranean formation usingsuch cement compositions. One or more flexible, compressible beads aremixed with the cement before pumping the cement slurry into a well bore.The flexible, compressible beads are preferably composed of anelastomeric material such as a copolymer of methylmethacrylate andacrylonitrile; a terpolymer of methylmethacrylate, acrylonitrile, andvinylidene dichloride; phenolic resins; a styrene-divinylbenzenecopolymer; and polystyrene. The flexible, compressible beads may beheated to expand the beads before mixing with the cement such that theensuing cement composition will have a desired density. Non-flexiblebeads such as glass hollow beads, cenospheres, and ceramic hollowspheres may also be added to the cement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the invention, a relatively lightweight cementcomposition is formed by combining flexible, compressible beads with ahydraulic cement and a fluid such as water. Any suitable flexible,compressible bead that may expand and contract and that is compatiblewith a cement (i.e., chemically stable over time upon incorporation intothe cement) may be combined with the cement to reduce its density.Flexible bead as used herein refers to a bead that may expand andcontract without adverse effect to the structure of the bead in responseto changes in pressure and/or temperature. Preferred flexible,compressible beads are substantially hollow objects filled with fluid(preferably gas), preferably spherical or substantially spherical inshape, and having a flexible outer wall. Preferred flexible,compressible beads have a diameter of about 6 to 150 micrometers at 25°C. and atmospheric pressure. Preferably, the fluid inside the flexible,compressible beads is air, carbon dioxide, an inert gas such asnitrogen, or an organic liquid with a low boiling point such asn-butane, isobutane or pentane. Preferably, the flexible, compressiblebeads have a substantially uniform, flexible outer wall comprising ofone or more elastomeric materials or polymers. The temperature at whichthe elastomeric material melts or becomes so soft that it loses itsability to contain the fluid and/or expand and contract is desirablyhigher than the temperature in the well bore, which may range from about120° F. to about 400° F. The elastomeric material is preferably astyrenic polymer, more preferably a copolymer of methylmethacrylate andacrylonitrile or a terpolymer of methylmethacrylate, acrylonitrile, andvinylidene dichloride. Flexible, compressible beads composed of thiscopolymer and this terpolymer are commercially available from AkzoNobel, Inc., which is located in Duluth, Ga., under the tradenameEXPANCEL. Several grades of EXPANCEL beads are available and may beselected depending upon the degree of expansion, the physical state, andthe temperature range for a given application. Other suitable materialsthat may be used to form the flexible wall include, but are not limitedto, a styrene-divinylbenzene copolymer and polystyrene. Hollowpolystyrene beads are available from many polystyrene suppliers, such asHuntsman Corporation of Houston, Tex. (sold as GRADE 27, GRADE 51, orGRADE 55) and BASF Corporation of North Mount Olive, N.J. (sold underthe tradename STYROPOR). The flexible, compressible beads areincorporated into the cement in a concentration of preferably from about1% to about 200% by weight of the cement (bwoc), more preferably fromabout 2% to about 100%, and most preferably from about 5% to about 50%.

In some embodiments, the flexible, compressible beads may be expandedbefore mixing with the cement by heating the flexible, compressiblebeads to soften the wall of the bead and to increase the pressure of thefluid (e.g., gas) therein. Preferred flexible, compressible beads arecapable of expanding up to 8 times their original diameters (i.e., thediameter at 25° C. and atmospheric pressure). For example, EXPANCELbeads having a diameter in the range of 6 to 40 microns, upon expansionincrease to a diameter of 20 to 150 microns. When exposed to heat, thebeads can expand up to forty times or greater their original volumes.The expansion of the beads is generally measured by the decrease in thespecific gravity of the expanded material. Thus, for example, whenEXPANCEL beads are heated to above 212° F., the density of the beadsdecreases from 1,000 grams per liter for the unexpanded beads to about30 grams per liter for the expanded beads. The temperature at which theflexible, compressible beads are heated depends on the polymercomposition of the bead wall and the desired density of the cementcomposition, which is typically in a range of from about 6 to about 23lb/gal. The flexible, compressible beads may be added to the cementcomposition by dry blending with the cement before the addition of afluid such as water, by mixing with the fluid to be added to the cement,or by mixing with the cement slurry consecutively with or after theaddition of the fluid. The beads may be presuspended in water andinjected into the cement mix fluid or into the cement slurry as anaqueous slurry. Surfactants may be added to the composition to water-wetthe surface of the beads so that they will remain suspended in theaqueous phase even if the density of the beads is less than that of thewater. The surfactants are preferably nonionic, with aHydrophile-Lipophile Balance values in the range 9-18. The ability of asurfactant to emulsify two immiscible fluids, such as oil and water, isoften described in terms of Hydrophile-Lipophile balance (HLB) values.These values, ranging from 0 to 40, are indicative of the emulsificationbehavior of a surfactant and are related to the balance betweenhydrophilic and lipophilic portions of the molecules. In general,surfactants with higher HLB values are more hydrophilic than those withlower HLB values. As such, they are generally more soluble in water andare used in applications where water constitutes the major or externalphase and a less polar organic fluid constitutes the minor or internalphase. Thus, for example, surfactants with HLB values in the range 3-6are suitable for producing water-in-oil emulsions, whereas those withHLB values in the 8-18 range are suitable for producing oil-in-wateremulsions. A commonly used formula for calculating HLB values fornonionic surfactants is given below:HLB=20×M _(H)/(M _(H) +M _(L))where M_(H) is the formula weight of the hydrophilic portion of themolecule and M_(L) is the formula weight of the lipophilic portion ofthe molecule. When mixtures of surfactants are used, the overall HLBvalues for the mixture is calculated by summing the HLB contributionsfrom different surfactants as shown in equation below:HLB=({acute over (ø)}₁ ×HLB ₁+{acute over (ø)}₂ ×HLB ₂+ . . . + . . .etc.,)where {acute over (ø)}₁ is the weight fraction of surfactant # 1 in thetotal mixture, HLB₁ is the calculated HLB value of surfactant #1, {acuteover (ø)}₂ is the weight fraction of surfactant #2 in the totalsurfactant mixture, and HLB₂ is the calculated HLB value of thesurfactant #2, and so on.

It has been observed that a mixture of a preferentially oil-solublesurfactant and a preferentially water-soluble surfactant provides betterand more stable emulsions. In particular, non-ionic ethoxylatedsurfactant mixtures containing from about 4 to about 14 moles ofethylene oxide. The HLB ratio for a single surfactant or a surfactantmixture employed in the present invention preferably ranges from about 7to about 20, more preferably from about 8 to about 18.

In one embodiment, a cement slurry densified by using a lower water tocement ratio is lightened to a desired density by the addition ofunexpanded or pre-expanded flexible, compressible beads in order to makethe final cement less brittle.

In another embodiment, hollow, non-flexible beads are mixed with thecement and the flexible, compressible beads. Particularly suitablenon-flexible beads are cenospheres, which are commercially availablefrom, for example, PQ Corporation of Valley Forge, Philadelphia underthe tradename EXTENDOSPHERES, from Halliburton Energy Services Inc.under the tradename SPHERELITE, and from Trelleborg Fillite Inc. ofAtlanta, Ga. under the tradename FILLITE. Alternatively, thenon-flexible beads may be glass beads or ceramic beads. The non-flexiblebeads, particularly the industrial waste product of the cenosphere type,are relatively inexpensive as compared to the polymeric flexible,compressible beads. However, the non-flexible beads are more likely tobreak when subjected to downhole temperature and pressure changes andprovide brittle cement compositions.

The presence of the flexible, compressible beads in the cementcomposition provides several benefits. For example, the flexible,compressible beads protect the ensuing hardened cement from experiencingbrittle failure during the life of the well even if some of thenon-flexible beads collapse. That is, the flexible wall and the gasinside of each bead contracts under pressure and expands back to itsoriginal volume when the pressure is removed, thus providing a mechanismfor absorbing the imposed stress. The absorption of energy by theflexible wall is expected to reduce the breakage of the more brittlebeads when such compositions are used. The flexible wall and theenclosed fluid also expand when the temperature in the well boreincreases, and they contract when the temperature decreases. Further,the flexible, compressible beads improve the mechanical properties ofthe ensuing cement, such as its ductility and resilience. Cementcomprising flexible, compressible beads gains the following beneficialphysical properties as compared to the same cement composition withoutthe flexible, compressible beads: lower elastic (Young's) modulus,greater plastic deformation, increased tensile strength, and lowerPoisson's ratio without significantly compromising other desirableproperties such as compressive strength.

In determining the relative amounts of flexible, compressible beads andnon-flexible beads to add to the cement composition to decrease itsdensity, the additional costs incurred by using the flexible,compressible beads should be weighed against the benefits provided byusing the flexible, compressible beads. For example, the amount offlexible, compressible beads added to the cement may be in the range offrom about 2% bwoc to about 20% bwoc, and the amount of non-flexiblebeads in the cement may be in the range of from about 10% bwoc to about100% bwoc.

Any known cement may be utilized in the present invention, includinghydraulic cements composed of calcium, aluminum, silicon, oxygen, and/orsulfur which set and harden by reaction with water. Examples of suitablehydraulic cements are Portland cements, pozzolana cements, gypsumcements, high alumina content cements, silica cements, and highalkalinity cements. The cement is preferably a Portland cement, morepreferably a class A, C, G, or H Portland cement, and most preferably aclass A, G, or H Portland cement. A sufficient amount of fluid is alsoadded to the cement to form a pumpable cementitious slurry. The fluid ispreferably fresh water or salt water, i.e., an unsaturated aqueous saltsolution or a saturated aqueous salt solution such as brine or seawater.The amount of water present may vary and is preferably selected toprovide a cement slurry having a desired density. The amount of water inthe cement slurry is preferably in a range of from about 30% bwoc toabout 120% bwoc, and more preferably in a range of from about 36% bwocto about 54% bwoc.

As deemed appropriate by one skilled in the art, additional additivesmay be added to the cement composition for improving or changing theproperties of the ensuing hardened cement. Examples of such additivesinclude, but are not limited to, set retarders such as lignosulfonates,fluid loss control additives, defoamers, dispersing agents, setaccelerators, and formation conditioning agents. Other additives thatmay be introduced to the cement composition to prevent cement particlesfrom settling to the bottom of the fluid are, for example, bentonite andsilica fume, which is commercially available from Halliburton EnergyServices Inc. under the tradename SILICALITE. Further, a salt such assodium chloride may be added to the cement composition when the drillingzone has a high salt content.

In preferred embodiments, a well cementing process is performed usingthe cement composition containing the flexible, compressible beads. Thewell cementing process includes drilling a well bore down to thesubterranean zone while circulating a drilling fluid through the wellbore. A string of pipe, e.g., casing, is then run in the well bore. Thedrilling fluid is conditioned by circulating it downwardly through theinterior of the pipe and upwardly through the annulus, which is locatedbetween the exterior of the pipe and the walls of the well bore. Thecement composition comprising flexible, compressible beads is thendisplaced down through the pipe and up through the annulus, where it isallowed to set into a hard mass. In alternative embodiments, the cementcomposition may be used for other projects such as masonry or buildingconstruction.

EXAMPLES

The invention having been generally described, the following examplesare given as particular embodiments of the invention and to demonstratethe practice and advantages hereof. It is understood that the examplesare given by way of illustration and are not intended to limit thespecification or the claims to follow in any manner.

Example 1

A cement slurry containing EXPANCEL flexible, compressible beads inaccordance with the present invention was formed by mixing together thefollowing components according to the procedure described in AmericanPetroleum Institute (API) Specification 10, 5th Edition, Jul. 1, 1990:class H cement, water (117.20% bwoc); SILICALITE silica fume (16.9%bwoc); bentonite (4.0% bwoc); HALAD-344 fluid loss additive availablefrom Halliburton Energy Services (0.5% bwoc); SCR-100 cement setretarder available from Halliburton Energy Services (0.3% bwoc); sodiumchloride (18% bwoc); and defoamer (0.025 gal/sk). A dry mixture ofSPHERELITE cenospheres available from Halliburton Energy Services (55%bwoc) and EXPANCEL 53 WU beads (10% bwoc), which are composed of acopolymer of methyl methacrylate and acrylonitrile having a softeningtemperature above 200° F., was added to the slurry with slow agitation.The slurry was subjected to a pressure of 4,000 psi in an autoclave tosimulate the breakage of the cenospheres under downhole conditions. Theslurry density values before and after pressurizing are reported inTable 1 below. A portion of the cement slurry was then poured into2″×2″×2″ brass cube molds and cured at 135° F. in a pressure chamberunder a pressure of 5,200 psi. Another portion of the slurry was pouredinto 1″×2″ cylindrical steel molds and cured at the same temperature.The compressive strengths were measured on 2″×2″×2″ molds using strengthtesting equipment manufactured by Tinius Olsen of Willow Grove, Pa.,according to American Society for Testing and Materials (ASTM) procedureC190-97. A load versus displacement study was performed on 1″2″cylinders using a MTS load frame instrument manufactured by MTS SystemsCorporation of Eden Prairie, Minn. without using any confining pressures(shown below in Tables 2 and 3).

Example 2

The procedure of Example 1 was followed except that the EXPANCEL 53beads were replaced with EXPANCEL 820 WU beads, which are composed of aterpolymer of methylmethacrylate, vinylidene dichloride, andacrylonitrile having a softening temperature above 167° F.

Example 3

The procedure of Example 1 was followed except that the EXPANCEL 53beads were replaced with EXPANCEL 551 WU beads, which are composed of aterpolymer of methylmethacrylate, vinylidene dichloride, andacrylonitrile having a softening temperature above 200° F.

Comparative Example 1

Conventional cement slurry containing SPHERELITE non-flexible beads butno flexible, compressible beads was formed by mixing together thefollowing components: class H cement, water (117.20% bwoc); SILICALITEsilica fume (16.9% bwoc); bentonite (4.0% bwoc); HALAD-344 fluid lossadditive (0.5% bwoc); SCR-100 cement set retarder (0.3% bwoc); sodiumchloride (18% bwoc); defoamer (0.025 gal/sk); and SPHERELITE beads (65%bwoc). The cement slurry was cured in the same manner as described inExample 1. The compressive strength and load versus displacementanalysis were also performed as described in Example 1. Note that thedensity values for the slurries in Examples 1-3 and that of the slurryin Comparative Example 1 are essentially identical considering theexperimental error in the method of measurement (see Table 1). TABLE 1Comparative Example 1 Example 2 Example 3 Example 1 SPHERELITE Beads,55   55   55   65   % bwoc EXPANCEL 53 10   — — — Beads, % bwoc EXPANCEL820 — 10   — — Beads, % bwoc EXPANCEL 551 — — 10   — Beads, % bwocSlurry Density, ppg @ 10.5 10.5 10.4 10.4 atm. Pressure Slurry Density,ppg 11.3 11.3 11.3 11.3 after pressurizing @ 4000 psi for 5 min.

The bulk mechanical properties of the cements in the examples andcomparative examples are shown below in Table 2: TABLE 2 CompressiveStrength @ 135° F. for 24 hrs, Poisson's Young's Modulus psi Ratioe(+6), psi Example 1 1820 0.193 0.763 Example 2 1920 0.196 0.768 Example3 2120 0.194 0.683 Comparative 1675 0.221 0.82 Example 1

As shown in Table 2, the compressive strengths of the cements containingboth flexible, compressible and non-flexible beads (Examples 1-3) aregreater than the compressive strengths of the cement containing onlynon-flexible beads (Comparative Example 1). The Young's Modulus valuesof the cements in Examples 1-3 are lower than the Young's modulus valueof the cement in Comparative Example 1, indicating that replacement of aportion of the brittle beads with flexible, compressible beads decreasedthe brittleness and improved the resiliency of the composition. Young'sModulus measures the interparticle bonding forces and thus the stiffnessof a material. As such, the cements in Examples 1-3 are less stiff thanthe cement in Comparative Example 1, which contains no flexible,compressible beads, and at the same time remain resilient up to higherstress levels.

This result is surprising because, in general, when a softer or a moreflexible (lower Young's modulus) material is added to a brittlematerial, the final composition has a lower compressive strength as wellas a lower Young's modulus. In the present case, even though the Young'smodulus decreased as expected, the compressive strength increased,suggesting synergistic interaction between the flexible, compressiblebeads and the non-flexible, brittle beads. Without being limited bytheory, it is believed that the stress imposed in a compressive mode isabsorbed effectively by the flexible, compressible beads, resulting inincreased load values at which the brittle beads, and thus the entirecomposition fails.

The mechanical properties at the yield points of the cements formed inExamples 1-3 and Comparative Example 1 were obtained from the load vsdisplacement data analysis. These mechanical properties are presentedbelow in Table 3: TABLE 3 Radial Axial Area Area Axial Strain @ Strain @Axial Under Under Young's Stress @ Yield, Yield Strain to Axial RadialPoisson's Modulus @ Yield, psi Microinch/ Microinch/ Radial Curve @Curve Ratio @ Yield, psi (avg) inch inch Strain ratio Yield @Yield Yielde(+6) Example 1 2115 1480 4760 3.22 6710 2550 0.31 0.44 Example 2 22151635 5190 3.17 7750 2930 0.43 0.43 Example 3 1580 1460 3960 2.71 41101810 0.37 0.40 Comparative 1760 3555 4390 1.24 5250 4440 0.42 0.49Example 1

As shown in Table 3, the radial strain values at yield (i.e., theelastic limit) for the cements in Examples 1-3 are much lower than theradial strain at yield for the cement in Comparative Example 1 due tothe compressible nature of the flexible hollow beads under pressure.Moreover, the axial strain to radial strain ratios of the cements inExamples 1-3 are higher than the axial strain to radial strain ratio ofthe cement in Comparative Example 1. Therefore, when axial pressure isimposed on the cement column in the well bore, the radial expansion issignificantly less for the cements containing both flexible,compressible and non-flexible beads as compared to the cement containingonly non-flexible beads because of the reduction in volume of thecements containing the compressible, flexible beads. A significantradial expansion under axial stress is expected for cements containingnon-flexible beads such as those described in Comparative Example 1 orin cements where water is used to decrease the density. The Poisson'sRatio and Young's Modulus values at yield for the cements in Examples1-3 tend to be lower than or comparable to those values at yield for thecement in Comparative Example 1, as shown in Table 2. The total areaunder a load vs displacement curve reflects the ability of a material toabsorb the imposed stress in the direction of displacement. Comparingthe areas under the radial curves for the cement compositions inExamples 1-3 and the cement compositions in Comparative Example 1indicates the unique advantage the addition of flexible, compressiblebeads provides to the cement composition. Due to their compressiblenature, the beads absorb the axial stress without having to distributethe stress in a radial direction. As a result, the radial dissipation ofimposed axial stress is significantly lower for the compositions inExamples 1-3 than for the compositions in Comparative Example 1. Thisresult clearly indicates that during the life of the well, the imposedstresses will be primarily absorbed by the flexible, compressible beadswithout requiring changed dimensions to the cement columns.

Example 4

EXPANCEL 53 WU beads were suspended in three times the volume of watercompared to that of the beads, and the resulting slurry was charged intoa cylindrical stainless steel can provided with a lid to which astirring paddle was connected. The slurry filled ¼ the available volumein the can after the lid was fitted. The can was then inserted into aheated water bath of a HOWCO cement consistometer manufactured byHalliburton Energy Services. The motor in the consistometer was turnedwhich rotated the metal can while holding the lid steady. After stirringthe assembly in this manner for a period of time at a desiredtemperature, the can was disassembled, and the expanded solid thereinwas filtered and dried in open air at ambient temperature. Thisprocedure was repeated at different heating temperatures and times toobtain expanded beads of different specific gravities. In particular,when EXPANCEL 53 WU beads of specific gravity 1.1 were heated at 170° F.for 4 hours, the specific gravity of the expanded beads was 0.345;whereas when the same material was heated to 200° F. for 4 hours, thespecific gravity of the expanded beads was 0.1.

A cement slurry having a density of 11.3 pounds per gallon was preparedaccording to the API procedure mentioned previously by mixing class Ccement with water (57% bwoc), SILICALITE fumed silica (15% bwoc), CFR-3dispersant supplied by Halliburton Energy Services (2% bwoc), theEXPANCEL 53 WU beads of specific gravity 0.3 pre-expanded as describedabove (9.8% bwoc), the EXPANCEL 53 WU beads of specific gravity 0.1pre-expanded as described above (2.6% bwoc), and a defoamer (2% bwoc).The slurry was poured into cylindrical plastic containers of dimensions2″×4″, closed with lids, and cured at room temperature for 24 hoursuntil the cement slurry solidified. The plastic containers weretransferred to a water bath kept at 180° F. for 18 hours, and thesamples were submitted to cyclical load/displacement studies using theequipment described in Example 1. The cyclic load/displacement studieswere performed by measuring the force to break an initial sample,followed by cycling the loads of subsequent samples between 20% and 90%of the load force to break the initial sample. When the load forcereached the maximum or minimum value, a two second resting time wasmaintained before the beginning of the next cycle. The axial and radialdisplacements were measured as a function of load force. The initialcompressive strengths were measured either under no confining pressure,or a confining pressure of 1000 psi. The results are shown in Table 4.

Comparative Example 2

Cement samples were prepared as described in Example 4 except that theEXPANCEL 53 WU beads were replaced with SPHERELITE cenospheres (25%bwoc). The slurry density was 12.6 pounds per gallon. The samples weresubmitted for cyclic load/displacement analysis. The results are shownin Table 4. TABLE 4 Confining Compressive Sample Pressure, psi Strength,psi # Cycles to Break Comparative None 6960 120 Example 2 Comparative1000 8000 200 Example 2 Example 4 None 3250 120 Example 4 1000 3300 240The results in Table 4 show that the composition containing the flexiblebeads lasted longer under cyclic loading and unloading of pressure underconfining conditions. The confining pressure is applied to simulate theconfinement on a cement column from the formation or another casingpipe.

Comparative Example 3

A cement slurry having a density of 12.02 pounds per gallon was preparedusing the API procedure mentioned in Example 1 by mixing class H cementwith water (54% bwoc), unexpanded hollow polystyrene beads of EPS(expandable polystyrene) grade, ethoxylated (10 moles) nonylphenol (0.04gallon per sack of cement), and a defoamer. The slurry was poured intocubic molds as described in Example 1 and cured in an autoclave at 155°F. for 24 hours under a pressure of 3,000 psi. The pressure wasreleased, and the density of the slurry was measured to be 12.3 poundsper gallon. The measured density of the slurry after curing underpressure was similar to the original slurry density, suggesting that thepolystyrene beads were essentially non-compressible.

Example 5

EPS grade hollow polystyrene beads of specific gravity 1.01 were heatedin water to 170° F. for 3 hours following the procedure described inExample 4. The expanded beads were filtered and dried. The specificgravity of the expanded beads was 0.1.

A cement slurry having a 12.08 pounds per gallon density was prepared asdescribed in Comparative Example 3 except that the unexpanded hollowpolystyrene beads were replaced with the pre-expanded polystyrene beadsof specific gravity of 0.1. The slurry was cured under the sameconditions as described in Comparative Example 3. The density measuredafter curing under pressure was 14.9 pounds per gallon, clearlyindicating that pre-expansion of the beads made them flexible andcompressible.

While the preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit and teachings of the invention. Theembodiments described herein are exemplary only, and are not intended tobe limiting. Many variations and modifications of the inventiondisclosed herein are possible and are within the scope of the invention.Use of the term “optionally” with respect to any element of a claim isintended to mean that the subject element is required, or alternatively,is not required. Both alternatives are intended to be within the scopeof the claims.

Accordingly, the scope of protection is not limited by the descriptionset out above, but is only limited by the claims which follow, thatscope including all equivalents of the subject matter of the claims.Each and every claim is incorporated into the specification as anembodiment of the present invention. Thus, the claims are a furtherdescription and are an addition to the preferred embodiments of thepresent invention. The discussion of a reference in the Description ofRelated Art is not an admission that it is prior art to the presentinvention, especially any reference that may have a publication dateafter the priority date of this application. The disclosures of allpatents, patent applications, and publications cited herein are herebyincorporated by reference, to the extent that they provide exemplary,procedural or other details supplementary to those set forth herein.

1. A cement composition comprising: hydraulic cement; and one or moreflexible beads.
 2. The composition of claim 1 wherein the beads arecapable of expanding up to about 8 times their original diameters. 3.The composition of claim 1 wherein the beads are capable of expanding upto about 40 times or greater their original volumes.
 4. The compositionof claim 1 wherein the beads are capable of contracting.
 5. Thecomposition of claim 1 wherein the beads comprise an elastomericmaterial selected from the group consisting of a copolymer ofmethylmethacrylate and acrylonitrile; a terpolymer ofmethylmethacrylate, acrylonitrile, and dichloroethane; astyrene-divinylbenzene copolymer; phenolic resins; and polystyrene. 6.The composition of claim 1 wherein the beads are hollow.
 7. Thecomposition of claim 6 wherein the beads contain a liquid.
 8. Thecomposition of claim 6 wherein the beads contain a gas.
 9. Thecomposition of claim 8 wherein the gas is selected from the groupconsisting of air, carbon dioxide, nitrogen, n-butane, isobutane,pentane, and combinations thereof.
 10. The composition of claim 1wherein the beads have a diameter ranging from about 6 to about 150micrometers at a temperature of 25° C. and at atmospheric pressure. 11.The composition of claim 1 wherein the beads are introduced to thecement composition in an amount in the range of from about 1% to about200% by weight of cement therein.
 12. The composition of claim 1 whereinthe beads are introduced to the cement composition in an amount in therange of from about 2% to about 100% by weight of cement therein. 13.The composition of claim 1 wherein the beads are introduced to thecement composition in an amount in the range of from about 5% to about50% by weight of cement therein.
 14. The composition of claim 1 whereinthe cement composition further comprises at least one of water,non-flexible beads, a surfactant, silica fume, bentonite, a fluid lossagent, a retarding agent, sodium chloride, and a defoamer.
 15. Thecomposition of claim 1 further comprising a surfactant or mixture ofsurfactants having an hydrophile-lipophile balance (HLP) of from about 7to about
 20. 16. The composition of claim 1 wherein the cementcomposition further comprises at least one of ceramic spheres, glassspheres, and cenospheres.
 17. The composition of claim 1 furthercomprising water in an amount sufficient to form a pumpable slurry. 18.The composition of claim 17 wherein the slurry has a density of fromabout 6 to about 23 lb/gal.
 19. The composition of claim 18 wherein theslurry comprises expanded hollow beads that contract upon curing of theslurry.
 20. The composition of claim 19 wherein the density of the curedslurry is greater than the density of the uncured slurry.